1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory element of which a charge storage layer is made of an insulating film.
2. Description of the Related Art
As one of memory structures related to a NAND type flash memory, a MONOS (metal/oxide/nitride/oxide/semiconductor) structure may be exemplified. With the MONOS structure, the interference between the adjacent cells can be suppressed.
The above-described MONOS structure of metal/oxide/nitride/oxide/semiconductor is a typical example. The MONOS structure is not limited in the structure of metal/oxide/nitride/oxide/semiconductor.
The MONOS type memory cell is characterized in that the charge storage layer is made of an insulating film. In the MONOS type memory cell, namely, a channel is formed between a source diffusion layer and a drain diffusion layer formed at a silicon substrate so that a tunnel insulating film, a silicon nitride film, a blocking film, and a gate are subsequently formed on the channel. In the writing/erasing operation, the charge is passed through the tunnel insulating film. The silicon nitride film functions as a charge storage film. The blocking film keeps a charge stored in the charge storage film.
Recently, the development of operation speed is required for the flash memory such as the MONOS type memory cell as described above. In this point of view, such an attempt is made as making the tunnel insulating film of the MONOS type memory cell of a silicon oxynitride (SiON) film with high dielectric constant and reducing the thickness of the SiON film so that the erasing speed can be developed and thus, the operation speed can be developed (Reference 1).
The development of capacity is also required for the flash memory in addition to the development of operation speed. In this point of view, such an attempt is made as making the charge storage layer of an insulating film with high dielectric constant (high-k) so as to increase the amount of charge to be stored in the charge storage layer instead of the silicon nitride film (Reference 2).
In a flash memory such as a MONOS type memory cell, however, the use of the insulating film with high dielectric constant can not develop the charge storage density of the flash memory sufficiently. The requirement of the development of memory capacity in the flash memory is not satisfied.
[Reference 1] JP-A 2004-165553 (KOKAI)
[Reference 2] T. Sugizaki, M. Kobayashi, M. Minakata, M. Yamaguchi, Y. Tamura, Y. Sugiyama, T. Nakanishi and H. Tanaka, “Novel Multi-bit SONOS Type Flash Memory Using a High-k Charge Trapping Layer,” Symp. VLSI Tech. Digest p. 27 (2003).